1. Field of the Invention:
The invention generally relates to a method for attaching a fused-quartz mirror to a metal substrate, and more particularly to a method in which one surface of a fused-quartz mirror is coated with a film of solder, of a class including pure indium solder, and a metal substrate is tinned with a film of indium eutectic alloy, consisting essentially of indium, bismuth, lead and tin and subsequently united with the mirror at a solder/alloy interface.
2. Description of the Prior Art:
A major contaminant found aboard spacecraft is water vapor. Water vapor, of course, is present as a consequence of it having been absorbed in or deposited on substances and structures found aboard spacecraft, or as a result of operations of fuel cells frequently included in spacecraft systems.
As can be appreciated by those familiar with the spacecraft industry, precise determination of spatial distribution and density of water vapor in spacecraft systems and experiments is important to successful operation.
Water vapor often is monitored by cooling a quartz crystal microbalance (QCM) to temperatures below 140.degree. K. and observing its mass loading rate. In a fully equipped laboratory, cooling of a QCM readily can be accomplished cryogenically using liquid nitrogen or helium. However, it is generally accepted that radiative cooling is potentially the most practical technique for cooling a QCM in a spacecraft environment, because of the general unavailability of cryogenic fluids aboard such craft.
Radiant coolers usually include a second-surface, fused-quartz mirror bonded to a metal substrate. Unfortunately, the coefficient of thermal expansion of a metal substrate, such as one formed of aluminum, copper and the like, is from fifteen to twenty-five times that of fused quartz. As a consequence, separation of a mirror from its substrate may be expected to occur.
In an effort to overcome the propensity of mirrors to separate from their substrate, radiant coolers have been fabricated utilizing spectrally selected paints and adhesive coatings of metallized plastics in forming the reflective surfaces. Moreover, attempts have been made to bond the mirrors to the substrates utilizing silicone rubber adhesives. Unfortunately, those materials heretofore employed in attempting to avoid the consequences of the differentials of coefficients of expansion which in operation occurs between the substrate and the mirror, have poor outgassing characteristics and thus tend to introduce inaccuracies in the monitoring of water vapor.
Use of thin layers of gold applied to each non-wettable surface of a pair of surfaces to be joined has been proposed for overcoming the aforementioned difficulties. When employing layers of gold for this purpose the layers are positioned in face-to-face contact, and a length of indium wire is placed along one edge of the interface between the layers and heated in a vacuum. The indium is thus caused to flow along the interface between the layers. For example, see the method disclosed in United States Letters Pat. No. 3,857,161. The disadvantages of such a technique is also discussed in the aforementioned patent. For example, it is pointed out that gold applied to the non-wettable surfaces, to serve as a wetting agent for the indium, is highly soluble in indium. Thus, unless a sufficiently thick film of gold is applied to the surface, it may completely dissolve in the molting indium, resulting in a de-wetting of the surfaces and a consequent failure of the seal. Additionally, the strength of the bond thus produced tends to be less than often is desired. Furthermore, it should be apparent that not only is the material utilized economically expensive, the technique is in itself rather complex.
Therefore, as can be appreciated, there currently exists a need for a practical method for bonding second-surface mirrors to radiant coolers which overcomes the aforementioned difficulties and disadvantages.
It is, therefore, the general purpose of the instant invention to provide a method for attaching a fused-quartz mirror to a conductive metal substrate and which has particular utility in the fabrication of radiatively-cooled QCM's for space application.